The lipooligosaccharide (LOS) of N. meningitidis plays an important role in meningococcal pathogenicity. Antibodies to oligosaccharide (OS) moiety of LOS induce bactericidal activities in vitro. We are preparing an LOS derived OS-protein conjugate as a vaccine that may prevent meningococcal disease. The OS was isolated from strain A1 LOS by acetic acid hydrolysis of the LOS and purified through P-4 chromatography. The carboxylic acid group of KDO in the OS was linked to a homobifunctional adipic acid dihydrazide (ADH) using carbodiimide (EDC) and N-hydroxysulfosuccinimide. This ADH derivatized OS was further coupled to tetanus toxoid (TT) using EDC. The molar ratio of OS to TT in three lots of conjugate preparations ranged from 10:1 to 18;1. Antigenicity of the conjugates in vitro was similar to that of LOS as determined by ELIZA in which a rabbit immune serum against whole cells was used as a binding antibody. Immunogenicity of the conjugates showed that the conjugates induced the specific IgG antibody to LOS in mice, and also induced the better antibody response in rabbits. MPL enhanced the immunogenicity of the OS-TT. The binding reactivities of 12 immunotype meningococcal LOS to the rabbit sera elicited by the OS-TT showed that M978 (B), 44/76 (B), and A1 (A) LOS had the strongest reactivity which indicated that the A1 OS-TT conjugate induced the immunotype 8 specific antisera against several group B strains especially two disease isolated strains 44/76 and BB431.